Acoustic sensors which employ microphones and which are used to determine direction of an acoustic wave source have been proposed for many applications, both in the military and commercial areas. One such system is an acoustic direction finder disclosed in U.S. Pat. No. 2,966,657 wherein the particular application of the apparatus is employed for locating the origin of sound waves due to gun fire and other similar sound-wave generating sources. Microphones or "cosine law" microphones are disclosed in an arrangement where signals of opposite polarity are produced when the signals are received from opposite directions in combination with an omnidirectional third microphone. Circuitry is provided to permit the determination of the arrival direction of sound waves through the processing of these three microphones.
In U.S. Pat. No. 2,459,162, acoustic apparatus is disclosed for determination of the direction of a sound wave source. A receiver is mounted on a rotatable shaft which then is employed in combination with a reflector such that there is a visual means controllable by the receiver and the reflector for indicating the direction of compressional or sound waves.
U.S. Pat. No. 3,854,117 discloses a passive system employing two or more microphones in a direction finding scheme. A phase difference detector is provided to receive first and second versions of two different sources of a common input signal to give a non-linear output indicating the phase differences. The reference clearly does not comprehend measurement of the closest point of approach since it is strictly designed to locate the direction of a sound source.
While all of the above-described systems are particularly complicated by the attention given to determining the precise angular location of the acoustic source with respect to the sensor it would be of substantial advantage to the art if a simpler sensor system could be provided which would determine the point in time when a source of acoustic energy is closest to the sensor. In the traffic control industry, it is often desireable to employe sensors which will cause traffic light signals or other traffic control means to operate in advance of the arrival of the vehicle at the point of control. By properly positioning a sensor so that the nearest point from a path of travel to the sensor is at a predetermined location in advance of the control area, detection of a vehicle which thus produces acoustic waves at this point will permit a signal to be passed to the point of control for operation by that means. Similarly, in the control of air traffic at airports, such as major airfields where a number of planes are taxiing simultaneously, it is often times appropriate to determine the location of certain taxiing vehicles so as to advise other traffic for coordinated operation of the runways. Automated control could be employed which would be activated by receipt of a signal from a sensor which could determine the passage of a particular sound generating taxiing airplane past the closest proximity to a given point.
Accordingly, it is an object of this invention to provide a sensor which is capable of determining the path of a moving source of acoustic energy to the extent that a real time determination of the closest approach point to the sensor by the vehicle is determined. Other objects will appear hereinafter.